1. Field of the Invention
The invention relates to the field of optical communication, and more particularly to a single mode fiber suitable for access network or long wavelength application and a method for producing the same. The fiber has excellent bending resistance and moderate effective area.
2. Description of the Related Art
In recent years, FTTx has become a hot topic in the field of fiber network construction and fibers suitable for FTTx have been widely studied. Low water peak single mode fibers insensitive to bending are popular. Conventional low water peak single mode fibers (in accordance with ITU-TG.652C/D) have a bending radius of 30 mm, so the use thereof in indoor and narrow environments is greatly restricted. When a long fiber is wrapped in a small storage box, it is exposed to great bending stresses. To meet the requirements of the FTTx network construction and apparatus miniaturization, bending resistant fibers are desired. In December 2006, ITU-T announced a new fiber standard (G.657 fibers): “Characteristics of a bending loss insensitive single mode optical fiber and cable for the access network”. Thus, to develop a bending resistant single mode fiber is of immense significance for promoting the development of optical fiber access technology.
It is well-known that the macro-bend loss of fibers is mainly related to the mode field diameter and cutoff wavelength thereof. Bending properties of fibers can be represented by an MAC value, which is defined as a ratio of mode field diameter at 1,550 nm wavelength to cutoff wavelength. The smaller the MAC value, the better the bending properties of fibers. Obviously, to reduce the mode field diameter or increase the cutoff wavelength can decrease the MAC value, which is achieved by slightly modifying the parameters of conventional matched-cladding single mode fibers (as shown in FIG. 1). US2007/007016A1, CN1971321A, and CN1942793A disclose the method. However, if the mode field diameter is too small, the splicing loss will be large when connecting to conventional single mode fibers and the launched power is limited. Meanwhile, considering the characteristics of FTTx, in order to transmit with the whole band, the cutoff wavelength should not exceed 1,260 nm. Thus, the increase of the cutoff wavelength is very limited. Numerical aperture (NA) can be increased by higher doping, but high doping causes the increase of attenuation. Therefore, the method is not good.
Another method to improve bending properties of fibers is to design a depressed cladding (as shown in FIG. 2), as disclosed in U.S. Pat. No. 5,032,001, U.S. Pat. No. 7,043,125B2, and CN176680. The design increases the numerical aperture (NA) of fibers without increasing the doping, thereby preventing the increase of attenuation. A better method to improve bending properties of fibers is to design a depressed outer cladding (as shown in FIG. 3), whose basic waveguide structure is disclosed in U.S. Pat. No. 4,852,968, U.S. Pat. No. 6,535,679B2, and CN1982928A. However, these documents only aim at reducing additional bending loss, not considering the long-term service life of fibers under small bending radius. In fiber links, particularly in FTTx links, due to multi-point bending and connectors, multi-path interference (MPI) is a common phenomenon. The measurement of MPI was disclosed by David Z.hen in 2009 at OFC/NFOEC (“Testing MPI Threshold in Bend Insensitive Fiber Using Coherent Peak-To-Peak Power Method”). The depressed outer cladding should be designed accurately, if too close to the core, once the core deviates at the fiber connection point, the MPI easily occurs; if too far from the core, the additional bending loss cannot be decreased.
When a fiber bends, the outside thereof is exposed to the tensile stress. The tensile stress is represented by the following formula:
  σ  =            E      ·      r              (              R        +                  C          th                +        r            )      wherein E represents young modulus of silica glass, R represents a bending radius, r represents the radius of a fiber, and Cth represents the thickness of a coating. Based on the formula and the bending radius, the tensile stress imposed on a fiber with a glass cladding diameter of 125 μm and an outer diameter of 250 μm is calculated, as shown in FIG. 4. For example, when the bending radius is decreased to 6.5 mm, the tensile stress imposed on the outer bending wall of the fiber is 0.69 GPa (100 kpsi), which reaches the common screentest tension of fibers. Bending easily causes fracture, thereby increasing the building and maintenance cost and affects the reliability of systems. Upon preparing a fiber or a preform, to achieve an expected refractive index profile so that when refractive index changes, the material compositions vary accordingly, different layers should have different thermal expansion coefficient, heat capacity, and glass transition temperature. In the process of fiber drawing, because of different heating and cooling rate, the residual stress is inevitably produced. The stress originated from different glass transition temperature is something like mechanical stress, which is temporary and can be removed by improving the fiber drawing process. The stress originated from different thermal expansion coefficient is permanent stress, which is hardly removed by improving fiber drawing process, but can be removed by designing appropriate material composition and structure. The appendix of fiber standard ITU-TG.657 briefly describes the prediction of fiber life. The service life of fibers is related to the dynamic stress corrosion susceptibility parameter (nd) thereof. Under identical bending radius and storage length, the higher the nd of fibers, the higher the mechanical reliability thereof. Thus, it is urgent to develop a full-solid fiber that meets the standards G.652 and G.657 and has low additional bending loss, stable mechanical properties, and uniform material composition.